1. Field of the Invention
The present invention concerns a method for reducing and correcting image distortions that occur in the creation of MR images of an examination subject that is arranged on a table that is moved continuously through the MR system during the creation of the MR images, wherein the cause of the distortions is the acquisition of MR data of an image at varying positions within the MR system. The invention is in particular (but not exclusively) suitable for the correction of nonlinearities of the gradient fields that are switched in addition to the basic magnetic field B0 during the image generation.
2. Description of the Prior Art
In magnetic resonance tomography (MRT), a gradient field that ideally is linear, is superimposed on a static magnetic field that ideally is uniform, in order to be able to detect the position of protons in a subject by the linear spatial change of its precision frequency. In the image reconstruction from the acquired raw data, it is now assumed that the entire field consists of the static magnetic field and that the gradient fields vary linearly. Any deviation from this linear change leads to an incorrect localization of the proton in the MR image. In MRT acquisitions with fixed table position, the localization errors manifest in deformation artifacts (“pixel shifts”) and local intensity fluctuations. The causes for the non-homogeneity of the static field or the nonlinearity of the gradient field can be system imprecisions, eddy currents, local shieldings of the protons (known as chemical shift artifacts) or susceptibility effects at tissue boundaries, for example. The linearity of the entire magnetic field typically depends on the position within the MR system. The basic magnetic field B0 is essentially constant and the gradient field is essentially linear within a spherical or cylindrical region around what is known as the isocenter of the magnet.
A known method for covering larger examination regions is to organize the slices of the MR images to be acquired into multiple groups of adjacent slices; to drive the table on which the examination subject is arranged through the MR system in steps such that the center of a group respectively comes to be situated in the isocenter of the magnet; and to acquire raw MR data from the group when the table is stationary. The individual groups are thus acquired with various table positions. The main disadvantage of this method is that adjacent slices that are associated with different groups are acquired at opposite ends of the isocenter. Discontinuities result at the edges in the assembled images due to the unavoidable inhomogeneities and nonlinearities, which hinders the diagnosis.
Furthermore, it is known to acquire MR images as the table is driven continuously through the MR system. This technique yields good results (i.e. images) when the images are acquired with what is known as a single shot technique in which the entire raw data space (longitudinal section) is acquired in a single excitation of the magnetization in a very short time. If the continuous table feed is used in connection with other imaging sequences in which k-space is not read out after a single excitation (for example in fast spin echo or gradient echo sequences), the data are necessarily acquired at different positions within the MR system, wherein these different positions have different inhomogeneities and nonlinearities. This can lead to artifacts such as ghosting or a blurring, wherein the artifacts are greater the higher the table speed.
U.S. Pat. No. 4,591,789 describes a method for correcting nonlinearities of the gradient coil. U.S. Pat. No. 6,967,479 generalizes this method for a measurement with continuous table movement. It is common to both methods that the correction factors (thus the deviation from the linearity at a specific position within the MR system) must be known before the measurement. The calculation of the correction factors is also executed in U.S. Pat. No. 4,591,789 for only a specific design of the gradient coil. The embodiment from U.S. Pat. No. 6,967,479 is limited to a measurement with continuous table movement in which the frequency coding runs in the movement direction of the table during the image acquisition.